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Kinetics vs Thermodynamics
Product Distributions

Understand why reactions can form different products depending on temperature and conditions: the competition between the fastest pathway and the most stable outcome.

Organic Chemistry 1

What Are Kinetics and Thermodynamics?

Kinetics
Studies the rate of a reaction: how fast products form. Governed by the activation energy \( E_a \).
Thermodynamics
Studies the stability of products: which product is lower in energy. Governed by \( \Delta G \).

The Energy Diagram Tells the Story

Reaction Coordinate Energy Reactant TS (kinetic)
Lower \( E_a \)
 Kinetic Product (less stable) TS (thermo) Thermo Product (more stable)
\(\Delta G_\text{thermo}\)

Why Does This Matter?

In organic chemistry, many reactions produce more than one product. The ratio of products depends on whether the reaction is under kinetic control or thermodynamic control.

1
Drug synthesis: controlling product distribution determines yield of the desired molecule.
2
Industrial chemistry: temperature and time adjustments save millions in reagent costs.
3
Exam strategy: understanding this distinction is essential for predicting major products on tests.

Kinetic Control: The Fast Path

Kinetic control favors the product that forms fastest. This product has the lower activation energy \( E_a \), meaning its transition state is easier to reach.

Conditions
Low temperature, short reaction time, irreversible conditions
Key Equation
\( k = Ae^{-E_a / RT} \): lower \( E_a \) means larger \( k \) (faster rate)

Thermodynamic Control: The Stable Path

Thermodynamic control favors the product that is most stable. This product has the lowest overall energy (most negative \( \Delta G \)).

Conditions
High temperature, long reaction time, reversible (equilibrium) conditions
Key Equation
\( \Delta G = \Delta H - T\Delta S \): most negative \( \Delta G \) wins at equilibrium

Kinetic vs Thermodynamic: Side by Side

Kinetic Control Thermodynamic Control
Favors Fastest-forming product Most stable product
Determined by \( E_a \) (activation energy) \( \Delta G \) (free energy change)
Temperature Low High
Reaction time Short Long
Reversibility Irreversible Reversible (equilibrium)
Product stability Less stable More stable

Classic Example: 1,2 vs 1,4 Addition

When HBr adds to 1,3-butadiene, two products form. The ratio depends on reaction conditions.

1,3-Butadiene + HBr C1 C2 C3 C4 1,2-Addition Product Br on C2 (kinetic) 1,4-Addition Product Br on C4 (thermo)
At \(-80^\circ\text{C}\): 80% 1,2-product (kinetic)
At \(40^\circ\text{C}\): 80% 1,4-product (thermodynamic)

How to Predict the Major Product

1
Identify possible products. Draw all products that could form from the reaction.
2
Compare stability. Determine which product is more thermodynamically stable (more substituted, conjugated, or lower energy).
3
Check the conditions. Low temperature and short time favor the kinetic product. High temperature and long time favor the thermodynamic product.
4
Consider reversibility. If the reaction is irreversible, the kinetic product dominates regardless of temperature.

Example 1: Enolate Formation

A ketone with two different \(\alpha\)-hydrogens can form two enolates. Which one dominates depends on the base and conditions.

2-Methylcyclohexanone Kinetic Enolate Less substituted side
LDA, \(-78^\circ\text{C}\), fast addition
 Thermo Enolate More substituted side NaOEt, reflux, equilibrium LDA (strong, bulky base) = kinetic control NaOEt (weaker base, heat) = thermodynamic control

Example 2: Sulfonation Reversibility

Electrophilic aromatic substitution offers another classic example. Sulfonation is reversible; nitration is not.

Nitration
Irreversible. The product distribution is determined by relative rates of formation: kinetic control always.
Sulfonation
Reversible. At high temperature, the thermodynamic product dominates. Low temperature gives kinetic product.

Example 3: Temperature Switches the Product

Consider HBr addition to 1,3-butadiene at two temperatures. Let's trace the numbers.

Reaction Coordinate SM TS1 1,2-product TS2 1,4-product
\(\Delta G\)

The Reversibility Requirement

Thermodynamic control is only possible when the reaction is reversible. If products cannot return to reactants, the first product formed is the final product: kinetic control by default.

Irreversible Only kinetic control possible Reversible Both controls possible Low T Kinetic High T Thermo

Connection to Hammond's Postulate

Hammond's Postulate states that the transition state of a reaction step resembles the species closest to it in energy. This helps explain why kinetic and thermodynamic products can differ.

1
For exothermic reactions, the transition state resembles the reactant. The kinetic product forms via the TS with the lowest \( E_a \).
2
For endothermic reactions, the TS resembles the product. In this case, the most stable product also has the lowest \( E_a \), so kinetic and thermodynamic products are the same.

Common Mistakes to Avoid

✕ Confusing Rate with Stability
A faster-forming product is not necessarily more stable. Rate depends on \( E_a \), stability depends on \( \Delta G \).
✕ Ignoring Reversibility
Thermodynamic control requires the reaction to be reversible. If it is irreversible, only kinetic control applies.
✕ Temperature Swap
Students often reverse the conditions. Remember: low T = kinetic, high T = thermodynamic.

Summary Table

Feature Kinetic Product Thermodynamic Product
Formation rate Faster Slower
Stability Less stable More stable
Activation energy \( E_a \) Lower Higher
\( \Delta G \) Less negative More negative
Favored at Low T, short time High T, long time
Classic example 1,2-addition product 1,4-addition product
Enolate example Less substituted (LDA) More substituted (NaOEt)

Question 1 of 5

Which factor primarily determines which product forms under kinetic control?

Question 2 of 5

HBr is added to 1,3-butadiene at \(40^\circ\text{C}\) with extended reaction time. Which product is the major product?

Question 3 of 5

Why is reversibility required for thermodynamic control?

Question 4 of 5

When 2-methylcyclohexanone is treated with LDA at \(-78^\circ\text{C}\), which enolate forms preferentially?

Question 5 of 5

According to Hammond's Postulate, when do the kinetic and thermodynamic products coincide (become the same product)?